This invention relates generally to the art of vacuum coating, and more particularly to the art of producing vacuum coatings which maintain metallic properties throughout high temperature processes such as tempering.
Many metallic coatings vacuum deposited on glass lose their characteristic metallic properties when subjected to high temperature processing. Vacuum coatings with metallic properties such as conductivity and infrared reflectance are generally metals, metal nitrides, metal carbides or metal borides, which oxidize when heated in air to form metal oxides which are electrically insulating, more transparent and less absorbing. While many metals can be heated in air to the forming temperature of glass (600 to 700.degree. C.) and develop a protective oxide surface layer, the thinness of transparent metallic coatings and their consequent non-bulk, even porous, nature prevent the formation of a suitable protective layer. Thus thin transparent metallic films generally cannot be heated to temperatures at which glass can be bent without degradation of metallic properties.
U.S. Pat. No. 4,992,087 to Holscher discloses a process for the production of a tempered or bent glass plate with a transmission-reducing coating in which to one side of the glass plate is applied at least one opaque metal coating predominantly at least one metal or alloy of elements 22 to 28, and a metal-containing protective coating of an alloy of aluminum and at least 10 atomic percent titanium and/or zirconium and thickness selected such that during tempering or bending there is no significant oxygen diffusion to the metal coating.
U.S. application Ser. No. 07/768,791 entitled "HEAT PROCESSABLE METALLIC VACUUM COATINGS" filed Sep. 30, 1991, by Gillery discloses that vacuum coatings with a metallic appearance as deposited can be made to retain their metallic appearance upon bending by overcoating with a different metal which forms a dense oxide, and that further improvement in oxidation resistance of the metallic film can be attained by introducing additional interfaces formed by another layer of a different material, particularly an amorphous metal oxide.
A titanium nitride coating has metallic properties that make it suitable for a durable solar control coating. By changing the coating thickness, the transmission and solar properties can be varied, and by adding the appropriate combination of dielectric layers, reflectance and color can be varied while maintaining chemical and mechanical durability.
Such coated articles have particular application in monolithic automotive glazing. When the coating is deposited on a dark substrate such as Solargray.RTM. glass it can be used for privacy glazing with enhanced solar properties and a desired reflectance and color. On clear glass the titanium nitride layer can be adjusted to allow for greater than 70 percent transmittance of Illuminant A (LTA) with low internal reflectance, neutral appearance and enhanced solar properties. However most vehicle transparencies are bent and tempered.
In order to use titanium nitride on a flat glass substrate which is subsequently bent and tempered, it has now been discovered that it must not only be protected from oxidation by means of a protective overcoat layer, but also must be stabilized, for example, against glass substrate-titanium nitride layer interaction or stress-induced "breakdown" which occur at the high temperatures required for tempering. For example, a coating of titanium nitride/silicon nitride made by a magnetron sputtering process, where the silicon nitride overcoat prevents oxidation of the titanium nitride, was found not to survive tempering for silicon nitride layer thicknesses up to 800 Angstroms. Such a coating becomes hazy, mottled, crazed and develops a picture frame effect (coating breakdown around the edge of glass plate) after tempering. In addition, the coating is susceptible to glass surface contamination, such as packer belt marks, washer contamination in the form of streaks, or spots in the coating after heating.